Drilling wells for oil and gas production conventionally employs a longitudinally extending “string” comprising sections of drill pipe with heavy walled drill “collars” at the end to which is secured a drill bit of a larger diameter than the pipe. After a selected portion of the bore hole has been drilled, a string of tubular members of lesser diameter than the bore hole, known as a casing string, is placed in the bore hole. Subsequently, the annulus between the wall of the bore hole and the outside of the casing string is filled with cement by pumping the cement down through a so-called “flat shoe” at the end of the casing and, in some instances, through apertures in cementing collars at intervals in the casing string. Therefore, drilling and running and cementing casing according to the conventional process typically requires sequentially drilling the bore hole using drill string with a drill bit attached thereto, removing the drill string and drill bit from the bore hole, and disposing and cementing a casing into the bore hole. Further, often after a section of the bore hole is lined with casing and cemented, additional drilling beyond the end of the casing string or through a sidewall of the casing string may be desired. In some instances, a string of smaller tubular members, known as a liner string, is run and cemented within previously run casing. As used herein, the term “casing” includes tubular members in the form of liners.
Unfortunately, sequential drilling and casing may be time consuming because, as may be appreciated, at the considerable depths reached during oil and gas production, the time required to implement complex retrieval procedures to recover the drill string may be considerable. Thus, such operations may be costly as well, since, for example, the beginning of profitable production can be greatly delayed. Moreover, control of the well may be difficult during the period of time that the drill pipe is being removed and the casing is being disposed into the bore hole.
Some approaches have been developed to address the difficulties associated with conventional drilling and casing operations and to increase efficiency. One such approach includes drilling with casing. Drilling with casing employs a drill bit, termed a “casing bit,” attached to the end of the casing string. U.S. patent application Ser. No. 10/783,720, assigned to the assignee of the present invention and the entire disclosure of which is incorporated herein by this reference, discloses various embodiments of casing bits and methods of drilling with casing. The casing bit functions not only to drill the earth formation, but also to guide the casing into the bore hole, and remains in place during cementing of the casing in place. The casing string is, thus, run into the bore hole as it is formed by the casing bit through application of weight-on-bit (WOB) and rotation of the casing string, eliminating the necessity of retrieving a drill string and drill bit after reaching a target depth where cementing is desired.
While this procedure greatly increases the efficiency of the drilling procedure, a further problem is encountered when the casing is cemented upon reaching the desired depth. While one advantage of drilling with casing is that the drill bit does not have to be retrieved from the well bore, further drilling may be required. For instance, cementing may be done for isolating certain subterranean strata from one another along a particular extent of the bore hole, but not at the desired depth. Thus, further drilling must pass through or around the drill bit attached to the end of the casing.
Drilling through the previous drill bit in order to advance may be difficult, as drill bits are required to remove rock from formations and, accordingly, often include drilling very resistant, robust structures typically manufactured from materials such as tungsten carbide, polycrystalline diamond, or steel. Attempting to drill through a drill bit affixed to the end of a casing may result in damage to the subsequent drill bit and bottom-hole assembly deployed or possibly the casing itself. It may be possible to drill through a drill bit or a casing with special tools known as mills, but these tools are unable to penetrate rock formations effectively and the mill would have to be retrieved or “tripped” from the hole and replaced with a drill bit. In this case, the time and expense saved by drilling with casing would have been lost.